Stock material or miscellaneous articles – Structurally defined web or sheet – Including components having same physical characteristic in...
Reexamination Certificate
1999-03-30
2002-06-25
Thibodeau, Paul (Department: 1773)
Stock material or miscellaneous articles
Structurally defined web or sheet
Including components having same physical characteristic in...
C428S349000, C428S461000, C428S515000, C428S516000, C264S173140, C264S173150, C264S173190
Reexamination Certificate
active
06410124
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to films with improved metallizable surfaces. More particularly, the present invention relates to a polymeric film with improved metallizable surfaces, wherein the film, particularly after metallization, possesses a superior balance of oxygen and water vapor barrier characteristics as well as optical properties.
BACKGROUND OF THE INVENTION
Polymeric films have been widely used for packaging various food and non-food products. In order to ensure proper preservation of products packaged in such polymeric films, it is necessary to provide films with barriers against the transmission of air, moisture, deleterious flavors, and the like. Also, in the packaging of various products, it is important to provide films having good optical properties. Conventional polymeric films, however, typically lack the balance of excellent gas barrier characteristics and good optical properties required for packaging films. For example, polypropylene films are particularly preferred in the manufacture of packaging films as a result of their low cost and ease of manufacture. Such films, however, inherently permit the transmission of oxygen and water vapor from the outside to the inside of the package. As will be recognized by those skilled in the art, transmission of oxygen and water vapor through food packaging promote deterioration of the foods packaged therein.
Multi-layer polymeric films having improved gas and moisture barrier characteristics have been developed. For example, it is known that films incorporating ethylene-vinyl alcohol copolymer as a coextruded or coated layer thereon exhibit improved oxygen and moisture barrier properties. U.S. Pat. No. 5,192,620 discloses an oriented polypropylene base layer having a coating on one surface of a blend of vinyl alcohol homopolymer or copolymer and an ethylene-acrylic acid copolymer, with a further metal layer thereon. Films incorporating ethylene-vinyl alcohol as coextruded or coated layers, however, are difficult and expensive to manufacture.
U.S. Pat. No. 5,194,318 discloses a metallized oriented thermoplastic film structure having a propylene homopolymer or copolymer substrate with a high density polyethylene skin layer on at least one side of the substrate. The high density polyethylene skin layer is further provided with a thin metal layer which has been vapor deposited thereon. In one embodiment, the high density polyethylene skin layer is treated with flame or corona discharge, and the metal coating is thereafter deposited onto the flame or corona discharge treated layer. Although films which incorporate a high density polyethylene skin do not suffer from the deleterious effects of metal pick off, such films do not provide optimum oxygen and water vapor barrier properties.
U.S. Pat. No. 5,254,394 discloses a heat-sealable multi-layer film which comprises: (a) a base layer comprising isotactic polypropylene; and (b) at least one additional layer comprising syndiotactic polypropylene. According to the patent, the film has, inter alia, both good heat sealing and optical properties.
While the prior art discloses various packaging films, many conventional films do not provide a surface which is desirable for metallization and do not possess balanced optical properties and oxygen and water vapor barrier characteristics. Some films provide heat sealing skin layers that are not adequate for metallizing. Thus, there is a need in the art for films fulfilling the requirement of improved surface for metallizing.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a film with improved metallizable surfaces.
It is also an object of the present invention to provide a film having excellent oxygen and water vapor barrier properties as well as good optical properties.
These and other objects of the present invention are achieved by providing a multi-layer film comprising:
(a) a base layer comprising a polyolefin; and
(b) at least one additional layer comprising a metallocene-catalyzed syndiotactic polypropylene, wherein the base layer and the at least one additional layer comprising a metallocene-catalyzed syndiotactic polypropylene have slow crystallization rates and are not suitable for use as a heat sealable film, unless an additional heat sealable layer is provided as an unrelated optional layer.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention relates to a multi-layer film including (a) a base layer comprising a polyolefin and (b) at least one additional layer comprising a metallocene-catalyzed syndiotactic polypropylene, wherein the base layer and the at least one additional layer comprising a metallocene-catalyzed syndiotactic polypropylene are not suitable for use, without unrelated optional layers, as a heat sealable film.
By the phrase “metallocene-catalyzed syndiotactic polypropylene,” it is meant that the syndiotactic polypropylene possesses a molecular structure that corresponds or substantially corresponds to a syndiotactic polypropylene obtained by using an appropriate metallocene catalyst in the polymerization of propylene. By the phrase “not suitable for use as a heat sealable film,” it is meant that the layer does not exhibit substantial heat sealability and/or hot tack in the temperature range of about 100 to about 130° C. Specifically, films may or may not exhibit a heat seal by standard tests at 100° C., with most values at 0 g/inch seal strength but, in all cases, hot tack measurements will be less than the minimum level accepted as a standard for use, i.e. 150 g/inch, at all heat sealing temperatures below 115° C. The multi-layer films of the present invention have improved metallizable surfaces and exhibit excellent gas (e.g., oxygen and water vapor) barrier properties, as well as good pick off, and optical properties (haze and gloss), when metallized.
The syndiotactic polypropylene polymers used to produce an excellent surface for metallizing have a very slow crystallization rate. This rate is so slow as to produce unacceptable hot tack and to give the polymers unacceptable sealing characteristics for heat sealable film applications. These polymers have different characteristics from the characteristics inherent in films with good heat sealability.
The polyolefin of the base layer contemplated for the present invention may be any suitable polyolefin useful in the manufacture of thermoplastic polymeric films. Preferably, the polyolefin of the base layer is a propylene homopolymer or copolymer. Propylene homopolymers of particular use include isotactic polypropylene, preferably 80-100% isotactic polypropylene, most preferably about 95% isotactic polypropylene. The propylene homopolymers preferably have a melt flow (measured in accordance with the standard ASTM D1238 method) ranging from about 1.2 to about 10 g/10 minutes, most preferably from about 2.5 to about 6 g/10 minutes. Particularly preferred propylene copolymers include 99.8-93/0.2-7 wt % propylene/ethylene copolymers and other copolymers.
Another desirable polymer suitable for the base layer of the film of this invention is a high density polyethylene (HDPE), which is a substantially linear polymer having a density, e.g., of 0.952 to 0.962 g/cc, a melting point of, e.g., 120 to 148° C. and a substantial degree of crystallinity.
If it is desired to produce a film which is opaque after being subjected to uniaxial or biaxial orientation, microspheres, incompatible polymers, and/or other adjunctives known in the art may optionally be dispersed in the base layer before extrusion and orientation of the film. Such adjunctives are typically material with a higher melting point than and immiscible with the polymer of the base layer and would include any of the polymers disclosed in, e.g., U.S. Pat. Nos. 4,377,616 and 4,632,869. The adjunctives may comprise a polymer, e.g., a polyester such as polybutylene terephthalate (PBT) or polyethylene terephthalate (PET), a nylon, an acrylic resin, polystyrene, or an inorganic material, such as synthetic or naturally-occurring compounds and miner
ExxonMobil Oil Corporation
James Rick F.
Santini Dennis P.
Thibodeau Paul
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